Heating element comprising chromium alloyed molybdenum disilicide and the use thereof

ABSTRACT

The present disclosure relates to a heating element, wherein at least one part of the heating element is manufactured from a molybdenum disilicide composition and wherein in the molybdenum disilicide composition, molybdenum is substituted by chromium according to (Mo1-xCrx)Si2 and x is in the range of 0.16≤x≤0.19.

TECHNICAL FIELD

The present disclosure relates to a heating element composed at leasttwo parts which are based on different molybdenum disilicide-basedcompositions, wherein at least one of the molybdenum disilicide-basedparts is based on a molybdenum disilicide composition in which part ofthe molybdenum is substituted by chromium according to(Mo_(1-x)Cr_(x))Si₂ and x is in the range of from 0.16≤x≤0.19 andwherein at least one part of the heating element is based on anothermolybdenum disilicide-based composition. The present disclosure alsorelates to the use of said heating element and to a furnace comprisingsaid heating element.

BACKGROUND

Molybdenum disilicide based materials have successfully been used inmany demanding high temperature applications, such as in parts inengines, turbines and furnaces. These materials typically exhibit goodmechanical properties at high temperatures, up to 1800° C., as well asgood corrosion and oxidation resistance in air, mainly owing to theformation of a continuous and well-adherent SiO₂ layer protecting themolybdenum disilicide.

However, heating of molybdenum disilicide based materials in air alsoleads to the formation of MoO₃ which, especially in the temperaturerange of 400-600° C., will disturb the formation of a continuous andwell-adherent SiO₂ layer on the molybdenum disilicide based material.This phenomenon was first described and termed “pesting” by Fitzer in1955. Since pesting hinders the formation of a protective silica layer,material consumption due to oxidation and corrosion will be both highand continuous where pesting has occurred. In a high temperatureapplication, such as a furnace, at least part of the heating elementsused therein will be in the pesting temperature regime.

It has been shown by for example Ström et al. in “Low temperatureoxidation of Cr-alloyed MoSi₂”, Transaction of Nonferrous Metals Societyof China, 2007: 17(6) 1282-1286 that chromium alloyed molybdenumdisilicide compositions such as (Mo_(0.90)Cr_(0.10))Si₂ and(Mo_(0.85)Cr_(0.15))Si₂ display an improved resistance towards pestingcompared to pure MoSi₂.

However, there still exists a need for new heating elements comprising amolybdenum disilicide based materials which will provide an improvedoxidation resistance.

SUMMARY

One aspect of the present disclosure is to provide a heating elementwhich will solve or at least reduce the above-mentioned problems and/orneeds.

The present disclosure therefore relates to a heating element composedof at least two molybdenum disilicide-based parts,

-   -   wherein at least one part is based on a molybdenum disilicide        composition comprising more than 90 weight % of        (Mo_(1-x)Cr_(x))Si₂ and wherein x is in the range of from        0.16≤x≤0.19;    -   and    -   wherein at least one part is based on a molybdenum disilicide        composition comprising        -   a) more than or equal to 90 weight % MoSi₂, balance is            aluminosilicate and/or SiO₂ or        -   b) more than or equal to 90 weight % (Mo,W)Si₂, balance is            aluminosilicate and/or SiO₂.

The present heating element will thereby have an improved resistancetowards pesting combined with good mechanical properties. Further, thepresent heating element will have high oxidation and corrosionresistance as well as good and reproducible mechanical properties andexcellent high temperature performance and will be suitable for hightemperature applications.

The heating element may be readily produced in various shapes and sizesand advantageously replace existing heating elements. Suitableapplications include, but are not limited to, heating arrangements forheating above 900° C.

The different parts of the heating element may be formed into rod orother forms and then connected. Furthermore, the parts may be shaped asU-elements but also as multi-shank, helical, diffusion cassettes, flatpanels, etc. The different parts may thus be in the form of rods and maybe bended or straight depending on the intended use of the heatingelement. The cross-section of the rod may typically be circular, butdepending on the application, other geometrical shapes may also bepossible such as elliptical or rectangular.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a graph illustrating the weight gain of different samplesas a function of exposure time at 450° C.;

FIG. 2 shows a graph illustrating the weight gain as a function ofexposure time at 450° C.;

FIG. 3 illustrates a heating element according to one embodiment of thepresent disclosure.

DETAILED DESCRIPTION

The present disclosure relates to a heating element composed of at leasttwo molybdenum disilicide-based parts,

-   -   wherein at least one part is based on a molybdenum disilicide        composition comprising more than 90 weight % of        (Mo_(1-x)Cr_(x))Si₂ and wherein x is in the range of from        0.16≤x≤0.19;    -   and wherein at least one part is based on a molybdenum        disilicide composition comprising    -   a) more than or equal to 90 weight % MoSi₂ balance is        aluminosilicate and/or SiO₂ or    -   b) more than or equal to 90 weight % (Mo,W)Si₂ balance is        aluminosilicate and/or SiO₂.

The range of chromium is of from 0.16≤x≤0.19, such as 0.16≤x≤0.18, suchas 0.165≤x≤0.175. This particular range of Cr has been found to furtherimprove the oxidation resistance of the heating elements and willalleviate the problems associated with pesting.

The present heating element is composed of at least one part which isbased on one molybdenum disilicide-based composition and at least onepart which is based on another molybdenum disilicide-based composition.As the parts will have different properties due to the composition onwhich they are based on, the heating element will also have differentproperties at different portions.

The part(s) of the heating element, which will be exposed to the coldzones (400 to 600° C.), of a furnace is based on the molybdenumdisilicide-based composition comprising more than or equal to 90 wt % of(Mo_(1-x)Cr_(x))Si₂ wherein x is of from 0.16 to 0.19. The balance ofthe composition may be aluminosilicate clay and/or one or more inorganicoxides, such as SiO₂. According to one embodiment, the aluminosilicateclay is of the montmorillonite type for example bentonite. It has beenshowed that a chromium alloyed molybdenum disilicide-based compositionwill not form molybdenum oxides in the cold zones, which means that thesilica dioxide layer formed will be continuous and therefore will not beexposed to degradation due to corrosion and/or wear. The part based onthe composition comprising (Mo_(1-x)Cr_(x))Si₂ may expand over to thehot zone(s) of the heating element and the part based on the compositioncomprising (Mo,Al)Si₂ may expand over in the cold zones of the heatingelement.

In the present disclosure, the terms “(Mo,Cr)Si₂-based material” and“(Mo_(1-x)Cr_(x))Si₂” and “a chromium-alloyed based molybdenumdisilicide” and “chromium-alloyed molybdenum disilicide-basedcomposition” are used interchangeably.

Furthermore, the part(s) of the heating element exposed to the heatzones (i.e. above 600° C.) is (are) based on (manufactured from) amolybdenum disilicide based composition comprising more than or equal to90 weight % of composition a) or b).

According to one embodiment, the chromium alloyed molybdenum disilicidecomposition comprising from 95 weight % (Mo_(1-x)Cr_(x))Si₂. Accordingto another embodiment, the balance of the chromium alloyed molybdenumdisilicide composition may be aluminosilicate clay and/or one or moreinorganic oxides, such as SiO₂. According to one embodiment, thealuminosilicate clay is of the montmorillonite type for examplebentonite. and the balance is 10 weight % or less bentonite and/or atleast one inorganic oxide.

According to one embodiment, compositions a) and b) may be used in thesame part of the heating element, i.e. the heating element may, besidesthe part(s) based on (Mo_(1-x)Cr_(x))Si₂, also comprise one or moreparts based on both molybdenum disilicide compositions a) and b).

The different parts of the heating element may either be joined(connected) directly to each other or they may be joined by usinganother part which will function as an intermediate material that canalleviate e.g. differences in thermal expansion coefficient of thedifferent parts. The parts of a heating element may be joined by usingwelding, such as diffusion welding or using by induction heating andthen subsequently applying an external pressure perpendicular to thejoint. An alternative is to pass an electrical current through the jointand then simultaneously apply external pressure perpendicular to thejoint.

A typical heating element is a two-shank U-shaped element, with aheating zone of the heating material of one diameter welded to terminalsof another diameter.

According to one embodiment, the heating element as defined hereinaboveor hereinafter comprises or consists of two parts of differentmolybdenum disilicide-based compositions. According to anotherembodiment, the heating element as defined hereinabove or hereinaftercomprises or consists of three parts, wherein two of the parts arecomposed of the same molybdenum disilicide-based composition. Accordingto another embodiment, the heating element as defined hereinabove orhereinafter comprises or consists of four molybdenum disilicide-basedparts wherein two parts are based on the chromium alloyed molybdenumdisilicide composition as defined hereinabove or hereinafter. Accordingto another embodiment, the heating element comprises or consist of twoparts based on the (Mo_(1-x)Cr_(x))Si₂ molybdenum disilicide-basedcomposition and one part based on the (Mo,Al) based composition.

Referring to the drawings, a heating element comprises a section knownas terminal(s) (see FIG. 1 ). The cold zone is located in this section.According to the present disclosure, the terminal is based on the partcomprising the chromium alloyed molybdenum disilicide-based compositionbut a small section of the terminal could also be made from the materialto be used in the hot zone. The heat zone section is preferablymanufactured from the other molybdenium disilicide composition. Theterminal may have a larger diameter than the heating zone. The terminalmay also be adapted to extend to the outside of the furnace through thefurnace wall and to be electrically connected on the outside of thefurnace.

FIG. 1 illustrates examples of a heating element according to thepresent disclosure. FIG. 1 discloses a heating element 1. The heatingelement 1 has terminals 2. Parts 3 of the terminals are composed ofchromium alloyed molybdenum disilicide composition and a part iscomposed of a molybdenum disilicide-based composition suitable for hotzone 4.

According to one embodiment, the part(s) based on the (Mo,Cr)Si₂-basedmaterial is (are) long enough to cover the zone(s) having a temperaturerange of 400-600° C. during operation. According to one embodiment, saidpart(s) is (are) in the form of a rod having a diameter of 1 to 30 mmand a length of 1 to 40 cm.

In the present description, the expression “the part is based on acomposition” is intended to mean that at least 70 weight % of the partis based on that composition.

The present disclosure is further illustrated by the followingnon-limiting example.

Example

Elemental powders of molybdenum, silicon and chromium were mixed andheated in argon gas to form (Mo_(1-x)Cr_(x))Si₂. The amount of Mo, Crand Si depended on the value of x. The obtained product (could bedescribed as a cake) (Mo_(1-x)Cr_(x))Si₂ was crushed and milled to anaverage particle size of 5 μm, followed by cold isostatic pressing at2000 bar in rubber moulds to form cylindrical green bodies. The greenbodies were sintered in argon for 1 hour at 1550-1600° C.

Several samples with varying chromium content were prepared according tothe method described above and their oxidation resistance wasinvestigated at 450° C. in air and compared with a reference sample ofpure MoSi₂ as well as samples having both lower and higher amounts ofchromium. Table 1 summarizes the samples used.

TABLE 1 Investigated samples Denotation Material in Figures Sample typeMoSi₂ MoSi₂ Reference sample (Mo_(0.88)Cr_(0.12))Si₂ Cr12 Comparablesamples (Mo_(0.87)Cr_(0.13))Si₂ Cr13 (Mo_(0.86)Cr_(0.14))Si₂ Cr14(Mo_(0.85)Cr_(0.15))Si₂ Cr15 (Mo_(0.80)Cr_(0.20))Si₂ Cr20(Mo_(0.84)Cr_(0.16))Si₂ Cr16 Samples according to(Mo_(0.83)Cr_(0.17))Si₂ Cr17 the present disclosure(Mo_(0.82)Cr_(0.18))Si₂ Cr18 (Mo_(0.81)Cr_(0.19))Si₂ Cr19

FIGS. 1 and 2 show the surprising and positive effects of substitutingmolybdenum with chromium in amounts according to the present disclosure.The FIGS. 1 and 2 plot the weight change of the samples as a function ofexposure time at a temperature of 450° C. in air for samples preparedaccording to the present disclosure compared to pure MoSi₂ andcompositions having both higher and lower amounts of chromium thancompositions according to the present disclosure. It is surprisinglyshown in FIG. 1 that the optimum amount of chromium according to thedisclosure is x=0.17. As can be seen from the figures, substitution withchromium in the amount of 0.16≤x≤0.19 has a positive effect on theoxidation resistance. The positive effect of substituting Mo with Cr inthe range of 0.16≤x≤0.19, is therefore clearly demonstrated in FIGS. 1and 2 .

Example 2

Mixtures of molybdenum, silicon and chromium powders were prepared andheated in Ar to form MoSi₂ and Mo_(0.85)Cr_(0.15)Si₂, respectively. Thereaction products were milled to an average particle diameter of 5 μm.Silicide powder was subsequently mixed with 5 wt. % bentonite (bentoliteL) and water to form a paste for extrusion. Respective composition wasextruded into 9 mm diameter rods, which were subsequently dried andpre-sintered in hydrogen for 1 h at 1375° C. Final sintering to achievefull density was then performed by resistance heating in air to 1500° C.for 5 minutes.

Samples of each composition were ground to remove the protective SiO₂scale that was formed during final sintering. Samples were placedindividually on alumina sample holders to collect potential oxidationproducts and include them in the weight measurements. The samples wereplaced in laboratory air in an electrical furnace heated to 450° C.employing FeCrAl heating elements and utilized with ceramic fiberinsulation. Sample and holder were weighted to monitor individual weightchanges as function of exposure time.

The combination (Mo,Cr)Si₂-based terminal portions on MoSi₂-basedportions together with MoSi₂-based heating zone material displayedsignificantly improved resistance.

The invention claimed is:
 1. A heating element composed of at least twomolybdenum disilicide-based parts, wherein at least one part is based ona molybdenum disilicide composition comprising more than 90 weight % of(Mo_(1-x)Cr_(x))Si₂ and wherein x is in the range of 0.16≤x≤0.19; andwherein at least one part is based on a molybdenum disilicidecomposition comprising a) more than or equal to 90 weight % MoSi₂balance is aluminosilicate and/or SiO₂ or b) more than or equal to 90weight % (Mo,W)Si₂ balance is aluminosilicate and/or SiO₂.
 2. Theheating element according to claim 1, wherein x is in the range of0.16≤x≤0.18.
 3. The heating element according to claim 1, wherein x isin the range of 0.165≤x≤0.175.
 4. The heating element according to claim1, wherein the molybdenum disilicide is substituted by chromiumcomprises from 95 weight % (Mo_(1-x)Cr_(x))Si₂.
 5. The heating elementaccording to claim 1, wherein the composition of molybdenum disilicideis substituted by chromium has a balance of aluminosilicate and/or SiO₂.6. The heating element according to claim 1, wherein an entirety of theheating element consists of the molybdenum disilicide composition. 7.The heating element according to claim 1, wherein the heating elementconsists of two parts.
 8. The heating element according to claim 1,wherein the heating element consists of three molybdenumdisilicide-based parts wherein two parts of the heating element arebased on the same molybdenum disilicide-based composition and one partof the heating element is based on a different molybdenumdisilicide-based composition.
 9. Use of a heating element according toclaim 1 in a furnace.
 10. A furnace comprising a heating elementaccording to claim 1.